I've invested considerable time into reviewing IDEs for ExtJS (it's time well spent) and thankfully the ideal choice of IDE is not quite so much of a murky world as it is with say Java.

The two killer features for productive development in my opinion are:

Autocomplete (or at least a good stab at in the javascript world)

In place documentation lookup

With that in mind WebStorm (or Intellij) are by far the best choices.

As with many of these tools the setup is key. WebStorm and Intellij will Index every file they can get their hands on so the memory footprint will be huge if you don't set them up correctly. Likewise inline documentation lookup needs to be set up correctly.

Below is a screen shot of my installation with both autocomplete and documentation lookup working as they should.

PyCharm

Did you guys consider PyCharm? I develop NoTex - a browser based editor for reStructuredText (available at https://notex.ch), and while the back-end is Python the front-end almost exclusively ExtJS.

While the indexing in PyCharm can take a while, if you tell your project *not* to including every crap sub-folder in your project, it runs really smooth and provides a nice intelli-sense for JavaScript/ExtJS (and Python). Definitely something I can recommend!